<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <form action="index.html">
        <input type="text" name="usname" id="">
        <input type="submit" value="登录" class="btn">
    </form>


</body>
<script>
    var usname = document.getElementsByName('usname');
    var btn = document.querySelector('.btn');
    var uname;
    // console.log(usname);
    btn.addEventListener('click', function () {
        uname = usname[0].value;
        console.log(uname);
    })
</script>

</html>